Various disorders and diseases exist which affect cognition. Cognition can be generally described as including at least three different components: attention, learning, and memory. Each of these components and their respective levels affect the overall level of a subject's cognitive ability. For instance, while Alzheimer's Disease patients suffer from a loss of overall cognition and thus deterioration of each of these characteristics, it is the loss of memory that is most often associated with the disease. In other diseases patients suffer from cognitive impairment that is more predominately associated with different characteristics of cognition. For instance, Attention Deficit Hyperactivity Disorder (ADHD), focuses on the individual's ability to maintain an attentive state. Other conditions include general dementias associated with other neurological diseases, aging, and treatment of conditions that can cause deleterious effects on mental capacity, such as cancer treatments, stroke/ischemia, and mental retardation.
The requirement of protein synthesis for long-term memory has been demonstrated over several decades for a variety of memory paradigms. Agranoff et al. (1967) Science 158: 1600-1601; Bergold et al. (1990) Proc. Natl. Acad. Sci. 87:3788-3791; Cavallaro et al. (2002) Proc. Natl. Acad. Sci. 99: 13279-16284; Crow et al. (1990) Proc. Natl. Acad. Sci. 87: 4490-4494; Crow et al. (1999) J. Neurophysiol. 82: 495-500; Epstein et al. (2003) Neurobiol. Learn. Mem. 79: 127-131; Ezzeddine et al. (2003) J. Neurosci. 23: 9585-9594; Farley et al. (1991) Proc. Natl. Acad. Sci. 88: 2016-2020; Flexner et al. (1996) Proc. Natl. Acad. Sci. 55: 369-374; Hyden et al. (1970) Proc. Natl. Acad. Sci. 65: 898-904; Nelson et al. (1990) Proc. Natl. Acad. Sci. 87: 269-273; Quattrone et al. (2001) Proc. Natl. Acad. Sci. 98: 11668-11673; Zhao et al. (1999) J. Biol. Chem. 274: 34893-34902; Zhao et al. (2000) FASEB J. 14: 290-300. Flexner originally showed that drug-induced inhibition of protein synthesis (e.g., with 5-propyluracil or anisomycin) blocked long-term memory when this inhibition occurred during a critical time interval following the training paradigm. Flexner et al. (1996) Proc. Natl. Acad. Sci. 55: 369-374. If protein synthesis was inhibited before this critical time window or at any time after this window, there was no effect on long-term memory. The identity of the proteins essential for memory consolidation, the mechanisms of their regulation, and their role in the consolidation of long-term memory has remained a mystery.
In many species the formation of long-term associative memory has also been shown to depend on translocation, and thus activation, of protein kinase C (PKC) isozymes to neuronal membranes. Initially, these PKC isozymes, when activated by a combination of calcium and co-factors, such as diacylglycerol, achieve a stable association with the inner aspect of the external neuronal membrane and membranes of internal organelle, such as the endoplasmic reticulum. PKC activation has been shown to occur in single identified Type B cells of the mollusk Hermissenda (McPhie et al. (1993) J. Neurochem. 60: 646-651), a variety of mammalian associative learning protocols, including rabbit nictitating membrane conditioning (Bank et al. (1988) Proc. Natl. Acad. Sci. 85: 1988-1992; Olds et al. (1989) Science 245: 866-869), rat spatial maze learning (Olds et al. (1990) J. Neurosci. 10: 3707-3713), and rat olfactory discrimination learning, upon Pavlovian conditioning. Furthermore, calexcitin (Nelson et al. (1990) Science 247: 1479-1483), a high-affinity substrate of the alpha isozyme of PKC increased in amount and phosphorylation (Kuzirian et al. (2001) J. Neurocytol. 30: 993-1008) within single identified Type B cells in a Pavlovian-conditioning-dependent manner.
There is increasing evidence that the individual PKC isozymes play different, sometimes opposing, roles in biological processes, providing two directions for pharmacological exploitation. One is the design of specific (preferably, isozyme specific) inhibitors of PKC. This approach is complicated by the fact that the catalytic domain is not the domain primarily responsible for the isotype specificity of PKC. The other approach is to develop isozyme-selective, regulatory site-directed PKC activators. These may provide a way to override the effect of other signal transduction pathways with opposite biological effects. Alternatively, by inducing down-regulation of PKC after acute activation, PKC activators may cause long term antagonism.
Following associative memory protocols, increased PKC association with the membrane fractions in specific brain regions can persist for many days (Olds et al. (1989) Science 245: 866-869). Consistent with these findings, administration of the potent PKC activator bryostatin, enhanced rats spatial maze learning (Sun et al. (2005) Eur. J. Pharmacol. 512: 45-51). Furthermore, clinical trials with the PKC activator, bryostatin, suggested (Marshall et al. (2002) Cancer Biology & Therapy 1: 409-416) that PKC activation effects might be enhanced by an intermittent schedule of drug delivery. One PKC activator, bryostatin, a macrolide lactone, activates PKC in sub-nanomolar concentrations (Talk et al. (1999) Neurobiol. Learn. Mem. 72: 95-117). Like phorbol esters and the endogenous activator DAG, bryostatin binds to the C1 domain within PKC and causes its translocation to membranes, which is then followed by downregulation.
The non-tumorigenic PKC activator, bryostatin, has undergone extensive testing in humans for the treatment of cancer in doses (25 μg/m2-120 μg/m2) known to cause initial PKC activation followed by prolonged downregulation (Prevostel et al. (2000) Journal of Cell Science 113: 2575-2584; Lu et al. (1998) Mol. Biol. Cell 18: 839-845; Leontieva et al. (2004) J. Biol. Chem. 279:5788-5801). Bryostatin activation of PKC has also recently been shown to activate the alpha-secretase that cleaves the amyloid precursor protein (APP) to generate the non-toxic fragments soluble precursor protein (sAPP) from human fibroblasts (Etcheberrigaray et al. (2004) Proc. Natl. Acad. Sci. 101: 11141-11146). Bryostatin also enhances learning and memory retention of the rat spatial maze task (Sun et al. (2005) Eur. J. Pharmacol. 512: 45-51), learning of the rabbit nictitating membrane paradigm (Schreurs and Alkon, unpublished), and in a preliminary report, Hermissenda conditioning (Scioletti et al. (2004) Biol. Bull. 207: 159). Accordingly, optimal activation of PKC is important for many molecular mechanisms that effect cognition in normal and diseased states.
Because the upregulation of PKC is difficult to achieve without downregulation, and vice versa, methods of upregulation of PKC while minimizing downregulation are needed to enhance the cognitive benefits observed associated with PKC activation. The methods and compositions of the present invention fulfill these needs and will greatly improve the clinical treatment for Alzheimer's disease and other neurodegenerative diseases, as well as, provide for improved cognitive enhancement prophylactically. The methods and compositions also provide treatment and/or enhancement of the cognitive state through the modulation of α-secretase.